Virtual impactor

ABSTRACT

Disclosed is a virtual impactor for separating particles of a predetermined size range from an aerosol, and for transporting the separated particles from the region they are separated from the aerosol to an analysis/collection region remote therefrom. The device comprises a conduit member having an inlet end and an outlet end. The conduit member is configured to define a substantially enclosed passageway between the inlet end and the outlet end. At least a portion of the conduit member is gas-permeable. The outlet end of the conduit member is positioned adjacent to the analysis/collection region. The virtual impactor is moved relative to the aerosol to be sampled (or the aerosol is moved relative to the impactor). The virtual impactor utilizes an operating gas that is introduced into the impactor through the gas-permeable portion of the conduit member. The operating gas is then bifurcated into a counterflow fraction that exits the inlet end of the conduit member, and a transporting flow fraction that passes along the conduit member to the analysis/collection region. The counterflow fraction is readily adjustable to control the size of the particles entering the analysis/collection region. Since the operating gas is introduced into the conduit member through the permeable portion of that member, separated particles transported therein are prevented from colliding with the inner surface of the conduit member and are thus not lost or contaminated.

TECHNICAL FIELD

This invention pertains to virtual impactors for separating particlesfrom an aerosol for analysis.

BACKGROUND INFORMATION

Aerosols are a suspension of solid or liquid particles in a gas.Aerosols that include particles with diameters of 1 micrometer or largercan be characterized as coarse-particle aerosols. The particles incoarse-particle aerosols can be composed of mechanically produced solidparticles, mechanically produced spray droplets, or atmospheric clouddroplets.

To most effectively study the physical and chemical characteristics ofthe aerosol particles, it is necessary to separate the particles fromthe aerosol gas and transport the separated particles to a point wherethey can be either collected for later study or scrutinized in situ withsuitable analytical instruments. Often it is desirable to sample andstudy only a particle size or range of sizes of particles. For accuratesampling and analysis, it is also important that the particles to besampled are effectively separated, i.e., isolated from other aerosolparticles having sizes outside of the desired range. Furthermore, theseparated particles must be transported to the region where they arecollected or analyzed without colliding with the structure of thesampling device. Such collisions result in losses when the particlesadhere to the structure. Even if the particles collide but bounce offthe structure they may be contaminated through contact with thestructure.

In the past, devices known as impactors have been used for separatingparticles from the aerosol gas. Generally, impactors consist of animpaction plate and means for causing the aerosol to flow toward theplate. The direction of the aerosol flow is abruptly changed near theimpaction plate so that the particles with sufficient inertia willdeviate from the flow where it changes direction and impact upon theplate. The particles that collect on the plate are subsequentlycollected and/or analyzed.

The determination of which particles will deviate from the flow andimpact upon the plate depends essentially upon two parameters. Theseparameters are: the particle's inertia, as quantified by a parameterknown as the "stop distance" L; and the minimum radius of curvaturer_(c) of the streamlines of the aerosol gas flow in the region where theflow changes direction. The streamlines are lines in the gas flow of theaerosol which are everywhere parallel to the direction of flow at agiven instant. For spherical particles having a diameter of 1 micrometeror larger, the stop distance L is defined as follows:

    L=V.sub.i mBf

where

V_(i) =the impaction velocity, defined as the velocity of the particleat a distance upstream from the directional change of the aerosol flow.

m=the mass of the particle.

B=the mobility of the particle, defined as the particle velocity perunit of drag force under steady state conditions. In equation form:

    B=1/(6πηr.sub.p)

where

n=viscosity of the aerosol gas.

r_(p) =particle radius.

and

f=a correction factor for particles with upstream Reynolds numbers,Re_(i), greater than unity, in equation form:

f=3ε^(-3/2) Re_(i) ⁻¹ [Re_(i) ^(1/3) ε^(1/2) +arctan(Re_(i) ^(-1/3)ε^(-1/2))-π/2.

where

Re_(i) =2r_(p) ρV_(i) /η.

ρ=density of the gas.

ε≅1/6, a numerical constant.

It is known that when the ratio of the stop distance L to the radius ofcurvature r_(c) is much greater than unity for a particle, that particlewill deviate from the streamlines of the gas flow and impact upon anobject if one is present. The ratio of the stop distance L to the radiusof curvature r_(c) of the gas flow streamline is referred to as theStokes number, Stk. Therefore, in order to separate particles of a knownstop distance from a passing flow of the aerosol, the structure must bearranged so that the radius of curvature r_(c) of the streamlines of theaerosol flow is small enough near the impaction plate to create a Stokesnumber of greater than 1 for that particular particle.

One problem with conventional impactors is that the separated particleshave a tendency to bounce off the impaction plate and become reentrainedwithin the flow. Furthermore, conventional impactors are designed toaccumulate particles on the impaction plate prior to analysis. For someparticles, such as cloud droplets, this accumulation makes accurateanalysis of certain characteristics of these particles extremelydifficult. More particularly, cloud droplets are typically comprised ofa volatile solvent such as water containing various solutes such assulfuric acid, nitric acid, bisulfate compounds and others. Once thecloud droplets are separated and impact upon the impaction plate, theliquid solvent undergoes mixing and, usually, uncontrolled evaporation.Furthermore, the properties of the solutes (some of which, such asnitric acid, are volatile themselves) undergo rapid changes, therebymaking it practically impossible to capture and measure these soluteswith conventional impactors. Nonvolatile solutes may also undergophysical or chemical changes due to contamination through contact withthe impaction plate and/or other stolutes accumulated on that plate.

Some impaction devices, known as virtual impactors, are configured toavoid the problem of particles bouncing off the impaction plate andbecoming reentrained in the aerosol flow. These devices are known asvirtual impactors because there is no particle impaction with a solidobject at the point where particles separate from the flow. Rather, theseparated particles are directed from the point of separation to acollection region located away from the remainder of the aerosol flow.For example, in one type of virtual impactor such as that shown in U.S.Pat. No. 4,301,002, issued to Loo, the aerosol flow is directed from anozzle toward a spaced-apart hollow collection probe. Suction is appliedacross the space between the nozzle and the probe in order to divert aportion of the aerosol flow away from the probe. The diverted portion ofthe flow carries most of the small-inertia particles from the probe. Theremaining undiverted portion of the original aerosol flow (hereinafter"sampling flow") is directed into the collection probe. The largerparticles (the precise size of which depends upon the magnitude of theapplied suction) separate from the diverted flow and are carried by thesampling flow into the collection probe where they are collected on afilter located away from the flow diversion region.

Past virtual impactor designs may reduce the particle reentrainingproblem discussed with respect to conventional impactors, but since thesampling flow is merely an undiverted portion of the original aerosolflow, the collected particles will necessarily include somesmall-inertia particles in the same concentration as in the generalaerosol. As noted, unseparated small-inertia particles will adverselyaffect the accuracy of coarse particle analysis. Past virtual impactorsalso lack provisions for minimizing losses or contamination due toparticle collisions with the walls of the probe or conduit in which theseparated particles flow. Furthermore, these devices propose noeffective means of transporting and treating separated liquid dropletsso that the solvent and solute of that droplet can be accuratelyanalyzed in situ.

SUMMARY OF THE INVENTION

This invention is directed to a virtual impactor that includes theutilization of an operating gas for effective, easily controlledseparation of particles from an aerosol. Once separated, the particlesare transported to a remote region for in situ analysis or collection.The operating gas is controlled to effect minimal collision losses orcontamination of the transported particles within the structure of theimpactor. Furthermore, since all of the aerosol gas is diverted from theanalysis/collection region, the above-noted problem of some unseparatedsmall-inertia particles reaching that region is solved by thisinvention.

As another aspect of this invention, when cloud droplets are separatedfor analysis, the operating gas is heated in order to evaporate theliquid of the cloud droplets within the confines of the impactor. Thewater vapor produced is readily measured with a hygrometer. Otherfractions of the evaporated liquid droplet (e.g., gaseous or solidparticle solutes) can be analyzed within the confines of the devicethrough the use of suitable analytical instruments.

A virtual impactor formed in accordance with this invention particularlycomprises a conduit member having an inlet end and an outlet end anddefining a substantially enclosed passageway between the inlet andoutlet ends. At least a portion of the conduit member is gas-permeable.By either moving the device through the aerosol or forcing the aerosolpast the device, a portion of the aerosol particles are directed intothe inlet end of the conduit member. Impactor operating gas is forcedthrough the permeable portion of the conduit member into the passageway.The flow of the operating gas is controlled so that one fraction of thegas exits the passageway through the inlet end of the conduit member,and the remaining fraction of the operating gas is drawn through thepassageway to the outlet end of the conduit member. The fraction of thegas that is directed to the outlet end of the conduit member transportsthe relatively large-inertia particles to a region where they areanalyzed and/or collected. The fraction of the operating gas that exitsthe inlet end of the conduit member prevents small-inertia particlesfrom reaching the collection/analysis region. The size of the particlespermitted to pass into the transporting fraction of the operating gas isreadily controlled by adjusting the relative flow rates of the impactoroperating gas flow fractions. The introduction of the operating gasthrough the permeable conduit member creates a radially inward flow ofgas near the inner surface of the conduit member that tends to preventthe collision of separated particles against that surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a virtual impactor formedin accordance with this invention;

FIG. 2 is a diagram of one embodiment of a flow system usable with thevirtual impactor of FIG. 1;

FIG. 3 is a schematic cross-sectional view of a virtual impactor formedin accordance with this invention in place within a venturi;

FIG. 4 is a diagram showing the size distribution of particles of alaboratory-produced aerosol that were separated and treated by a virtualimpactor formed in accordance with this invention; and

FIG. 5 is an alternative embodiment of a virtual impactor formed inaccordance with this invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 respectively show a schematic cross-sectional view of apreferred embodiment of a virtual impactor 10 formed in accordance withthis invention and a preferred flow system 12 for controlling itsoperation. The depicted device is adaptable for any type of ground-basedor atmospheric aerosol sampling. For atmospheric aerosol sampling, thedevice can be mounted to an aircraft and flown through the aerosol(e.g., clouds).

The virtual impactor 10 comprises a tubular conduit member 14 having aninlet end 16 and an outlet end 18 (FIG. 2) and a central passageway 20extending therebetween. The inlet end 16 of the conduit member 14 facesthe aerosol to be sampled. The outlet end 18 of the conduit member isconnected to a return flow control valve 22 in the flow system 12, whichis described in more detail below.

The conduit member 14 is formed of thin, gas-permeable material. Thismaterial is preferably formed of sintered, stainless steel powder having100 micrometer pore diameters. Suitable material for this purpose ismanufactured by Mott Metallurgical Corp., Farmington, Conn. Othergas-permeable materials may be used for all, or a portion of, theconduit member. For example, a tube formed of permeablepolyetetrafluoroethylene would also be suitable. The material must berigid enough to withstand gas pressure applied against it (as describedmore fully below) without collapsing.

The conduit member 14 is surrounded by a concentrically disposed tubularhousing 26. One end of the housing 26 is contoured inwardly to join theinlet end 16 of the conduit member 14 and define the leading end 28 ofthe virtual impactor 10. The other end of the housing branches away fromthe conduit member and is connected to a hereinafter-described operatinggas control valve 30 in the flow system 12. The space between theconduit member 14 and the housing 26 defines a chamber 32.

The virtual impactor formed in accordance with this invention employs anoperating gas for both controlling aerosol particle entry into thepassageway 20 of the conduit member 14, and for transpoting thoseparticles through the conduit member to a region where they can becollected or analyzed in situ.

Turning now to the operating gas flow system shown in FIG. 2, thatsystem comprises a vacuum and pressure pump 34 interconnected betweenthe return flow control valve 22 and the operating gas control valve 30.The pump is connected and arranged to apply suction to the outlet end 18of the conduit member and to pressurize the chamber 32 with operatinggas drawn from a suitable source 36. The operating gas can be air or aninert gas such as helium, nitrogen or argon. The inert gases aredesirable for situations where the analytical instruments employedrequire the absence of oxygen. The operating gas can also be a mixturecontaining reagents such as ozone, chlorine or hydrogen.

An analysis/collection region 38 is incorporated into the systemadjacent to the outlet end 18 of the conduit member 14 and upstream ofthe return flow control valve 22. Separated particles are transported tothis region where they are available for analysis and/or collection.This region includes instruments suitable for collecting or measuringthe characteristics of the particular particles under study. Forexample, when liquid droplets are sampled, a nephelometer, a hygrometerand a condensation nuclei counter are included, as are filteringdevices, mass flow meters, temperature gauges, etc.

An operating gas treatment subsystem 40 is incorporated into the flowsystem between the pump 34 and the operating gas control valve 30. Thissubsystem includes filters and scrubbers for removing impurities in theoperating gas, which is supplied to the pump 34 from the operating gassource 36 and from the gas returned from the virtual impactor asdescribed below. If the aerosol particles to be studied are liquiddroplets, it has been pointed out that it is desirable in most cases toevaporate the liquid in order to measure the vapor of the solvent andliberate its solutes for separate collection and/or analysis. Thus aheater is included in the operating gas treatment subsystem 40 to heatthe operating gas for the evaporation process.

The operating gas, the flow of which is symbolized by F₁ in the figures,is pumped through the treatment subsystem 40 and into the chamber 32 ofthe virtual impactor. The flow rate of the operating gas is controlledby the operating gas control valve 30. From the chamber 32, theoperating gas passes through the permeable wall 24 of the conduit memberand is bifurcated into opposing flow fractions, namely a transportingflow fraction, symbolized by F₂ in the figures, which is drawn by pump34 back to the outlet end 18 of the conduit member through theanalysis/collection region 38; and a counterflow fraction, symbolized byF₃ in the figures, which flows outwardly from the inlet end 16 of theconduit member. The transporting flow fraction F₂ is controlled by thereturn flow control valve 22, and as long as that flow rate is less thanthe initial operating gas flow rate F₁, there will always be acounterflow fraction F₃. Increasing or decreasing the transporting flowrate F₃ will correspondingly decrease or increase the rate of thecounterflow fraction F₃. It is clear that if the transporting flow F₂ isadjusted to equal the operating gas flow F₁, then there will be nocounterflow fraction F₃ effecting particle separation.

Separation of aerosol particles for analysis with a virtual impactorformed in accordance with this invention generally comprises two steps.Firstly, particles from the aerosol are directed into the inlet end 16of the conduit member 14. These particles make up an aggregate samplehaving sizes (as quantified by their stop distances L) smaller than theminimum size of particle desired for collection and/or analysis (thislatter size is known as the "cut size"). Secondly, the counterflowfraction F₃ of the operating gas is adjusted to expel particles from thepassageway 20 that are smaller than the cut size.

For the initial separation of the aggregate sample of particles from theaerosol, the virtual impactor is moved relative to the aerosol in orderto create an impaction velocity V_(i). The leading end 28 of the virtualimpactor is contoured to direct the flow of the aerosol around it. For asubstantially hemispherically-shaped leading end, the minimum radius ofcurvature r_(c) of the aerosol streamlines 29 flow around the leadingend 28 of the impactor is approximately equal to the radius of theleading end of the impactor. Thus, for a virtual impactor having aradius of 1 centimeter at its leading end, particles having a stopdistance L greater than 1 centimeter (i.e., a Stokes number greaterthan 1) will deviate from the aerosol flow and be directed into thepassageway 20 of the conduit member 14. For example, for an impactionvelocity of 100 meters per second, liquid droplets having a radius of3.0 micrometers have a stop distance of approximately 0.6 centimeter.Thus, particles of this size or larger will enter the inlet end of theconduit member.

Once particles having Stokes numbers greater than 1 are directed intothe passageway of the conduit member 14 they must completely traversethe counterflow fraction F₃ of the operating gas in order to reach thetransporting flow fraction F₂ that will transport them to theanalysis/collection region 38. The length L_(c) of the counterflowfraction F₃ is the length between the point of division of transportingflow and counterflow fractions (i.e., the flow stagnation plane 42,FIG. 1) to the inlet end 16 of the conduit member. It is clear thatparticles must have a stop distance L at least as long as L_(c) in orderto reach the transporting flow fraction F₂. Assuming even distributionof the operating gas flow F₁ through the permeable wall 24 of theconduit member 14, and assuming that the portion of the wall 24 enclosedby the housing 26 has a constant length X, then the length L_(c) of thecounterflow fraction of flow relative to the length of the permeablewall can be approximated by the ratio of the counterflow rate F₃ out ofthe virtual impactor to the return flow rate F₂, or in equation form:

    L.sub.c /X=F.sub.3 /F.sub.1

    L.sub.c =(F.sub.1 -F.sub.2)/F.sub.19 ·X

From the above equation it is clear that by merely adjusting relativeflow rates F₁ and/or F₂, the counterflow length L_(c) can be changed.Accordingly, by changing the counterflow length L_(c), the number ofparticles that can traverse that distance L_(c) will correspondinglychange. Therefore, particle sample cut sizes can be changedinstantaneously by merely altering the flow rates F₁,F₂ without the needto alter the impaction velocity or the structure of the virtualimpactor. These flow rates can be adjusted during a sampling operation.

The magnitude of the flow of the counterflow fraction F₃ through thelength L_(c) will also affect the cut size. That is, an increasedcounterflow rate over a given distance L_(c) will increase the dragforce on the particle, thus reduce its velocity relative to F₃.Therefore the particle's stop distance L will be reduced. If the stopdistance L of the particle is so reduced that it cannot traverse thecounterflow distance L_(c), it will be expelled from the passageway 20of the virtual impactor. For example, experimentation has revealed thata virtual impactor having a 0.61 centimeter inlet opening and a length Xof 10 centimeters that was passed through an aerosol at 107 meters persecond, liquid droplet aerosol particles having less than approximately8 micrometer radii were expelled from the passageway by the counterflowfraction F₃ of the operating gas when the operating gas was supplied at20 liters per minute and controlled to provide 6.1 liters per minute ofcounterflow over a length L_(c) of 3 centimeters. It can be appreciatedthat under lab conditions and using a prepared coarse-particle aerosol,the virtual impactor formed in accordance with this invention can bereadily calibrated by one of ordinary skill in the art in order todetermine a suitable counterflow rate and distance L_(c) for any givencut size, operating gas flow rate F₁ and impaction velocity.

The ideal value for the transporting flow rate F₂ of the operating gaswill depend upon the flow rate needed by any of the collection devicesor analytical instruments used to measure the properties of the aerosolparticles. In any event, the use of a counterflow fraction F₃ will causeall of the aerosol gas to be diverted from the passageway 20 of theconduit members 14. Therefore there is no opportunity for small-inertiaparticles (i.e., particles smaller than the cut size) to pass into thecollection/analysis region via an undiverted portion of aerosol flow.

The virtual impactor formed in accordance with this invention willconcentrate the number of the aerosol particles for sampling, therebyconverting dilute, substantially immeasurable aersols into concentratedaerosols that are readily measurable. For example, a virtual impactorhaving a 0.9 centimeter diameter inlet and operated with an impactionvelocity of 100 meters per second will receive an inflow of 380 litersper minute of aerosol. If the transporting flow in that virtual impactoris 8.5 liters per minute, by simple mass conservation, the particlesfrom the inflow volume will be concentrated within the return flowfraction F₃ by a factor of approximately 45 (i.e., 380/8.5). Thus, forexample, if the number concentration of liquid droplets in an aerosol is100 per cubic centimeter, the number concentration of particles enteringthe analysis/collection region will be approximately 4,500 per cubiccentimeter (assuming no coagulation or fragmentation of particles in thevirtual impactor). This concentration makes measurement of theseparticles substantially more sensitive and accurate.

As noted earlier, particles separated from an aerosol by a virtualimpactor formed in accordance with this invention are transported to theanalysis/collection region by the transporting flow fraction of theoperating gas. Since the operating gas passes through the permeableportion of the conduit member 14, there is a radially inwardly directedportion of the flow near the inner surface 24 of the conduit member.This radial flow functions to prevent particle collisions against theinner surface as the particles are being transported to theanalysis/collection region 38. Preferably, the housing 26 (hence, theradially inwardly directed flow of the operating gas) completelysurrounds the permeable conduit member from the inlet end 16 to a pointthat is very near the analysis/collection region 38. Thus particlesentrained within the transporting flow fraction F₂ of the operating gaswill undergo substantially no losses or contamination prior to in situanalysis or collection.

FIG. 3 shows a virtual impactor 10' that is structurally identical tothe virtual impactor 10 described above adapted to reside within aventuri 44. The virtual impactor is fixed inside the throat 46 of theventuri. Space is provided between the virtual impactor and the wall ofthe venturi. The outlet of the venturi (not shown) is connected to ablower. This configuration permits the creation of an impaction velocityin the laboratory setting using a laboratory prepared aerosol. Thisconfiguration is especially useful for calibrating the impactor, or forground-based sampling of aerosols such as high-humidity smog.Experiments were performed with this embodiment using a heated operatinggas to evaporate a laboratory-prepared aerosol having liquid droplets asparticles. Specifically, the venturi blower was adjusted to create animpaction velocity V_(i) of 115 meters per second at the venturi throat46. An aerosol having droplets between 5 and 40 micrometers in diameterwas prepared using a 10⁻⁴ molar solution of ammonium sulfate. Theoperating gas of the virtual impactor was adjusted so that dropletssmaller than approximately 16 micrometers in diameter were expelled bythe counterflow fraction of the gas. Droplets larger than approximately16 micrometers in diameter were evaporated by the heated transportinggas; the residual solute particles were then collected and measured.FIG. 4 shows the distribution of the residual particles; wherein Dp isthe particle diameter in micrometers, and dV/d log Dp is the volumedistribution function in terms of cubic micrometers per cubiccentimeter.

A virtual impactor formed in accordance with this invention is notrestricted to the tubular configuration described above. For example,with reference to the alternative embodiment of a vertical impactor 10"shown in FIG. 5, the conduit member 14" can be formed by a pair of flatparallel plates 15 joined at the edges and enclosed within acorrespondingly shaped housing 26" to form a substantially slotted oroblong-shaped inlet opening 16". The plates 15 are formed of the samegas-permeable material described with respect to the embodiment shown inFIG. 1. This alternative embodiment would permit a relatively largevolume of aerosol to be sampled or collected depending upon the lengthof the inlet end 16" of the conduit member 14". The operation of thisembodiment is substantially the same as described with respect to thetubular shaped virtual impactor.

It is contemplated that the virtual impactor formed in accordance withthis invention is applicable for separation of many types of aerosolparticles. For example, airborne microorganisms, pollens, etc. havingsufficient stop distances could be collected for analysis with thisdevice. Furthermore, the use of a virtual impactor formed in accordancewith this invention is not necessarily limited to sampling of aerosolparticles. For example, it is also contemplated that such a virtualimpactor could be readily adapted for use in processing certainproducts. For example, any liquid or semisolid solution of matter(coffee, radioactive material, pharmaceutical mixtures, etc.) that isamenable to conventional atomizing spray processes could be directedthrough the virtual impactor for separation and dehydration when theoperating gas is sufficiently heated. Besides dehydration, chemicaltreatments can be applied to the separated particles, such asdisinfection through use of any suitable oxidizing agent (such aschlorine gas) as the operating gas of the virtual impactor. Thus, whilethe invention has been described with reference to preferredembodiments, it is to be clearly understood by those skilled in the artthat the invention is not limited thereto. Rather, the scope of theinvention is to be interpreted only in conjunction with the appendedclaims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
 1. A device for separating particles from an aerosol where one of the device and the aerosol is in motion relative to the other and for transporting the separated particles, comprising:(a) a conduit member having an inlet end and an outlet end and defining a substantially enclosed passageway between its inlet and its outlet end, at least a portion of the conduit member that defines the passageway being gas-permeable, the passageway being adapted to receive at least a portion of the aerosol's particles through the inlet end as a result of the relative motion of the device and the aerosol; and (b) operating flow means associated with the conduit member for directing a gas through the gas-permeable portion of the conduit member and into the passageway, wherein a first fraction of the gas that is directed into the passageway exits the conduit member through the inlet end thereof, and wherein a second fraction of the gas that is directed into the passageway exits the conduit member through the outlet end thereof, a flow rate of the first fraction of the gas being adjustable to prevent passage through the passageway of some of the portion of the aerosol's particles, wherein aerosol particles received in the inlet end of the conduit member and not prevented from passing through the passageway by the first fraction of the gas are transported by the second fraction of the gas to the outlet end of the conduit member.
 2. The device of claim 1, wherein the operating flow means includes a housing surrounding the conduit member, the housing being configured to define a chamber between the conduit member and the housing; and wherein the operating flow means is constructed so that the chamber is pressurized with the gas that is directed through the permeable portion of the conduit member into the passageway.
 3. The device of claim 2 wherein the conduit member is substantially tubular and wherein the housing is substantially tubular.
 4. The device of claim 2 wherein the inlet end of the conduit member is substantially oblong in cross section.
 5. The device of claim 1 wherein substantially all of the conduit member is gas-permeable.
 6. The device of claim 1 further including gas treatment means for filtering, scrubbing, and heating the gas that is directed into the passageway of the conduit member.
 7. The device of claim 6 wherein the gas treatment means includes gas cleaning means for cleaning the gas, and heating means for heating the gas.
 8. A device for separating particles from an aerosol where one of the device and the aerosol is in motion relative to the other comprising:(a) a conduit member having an inlet end and an outlet end and defining a substantially enclosed passageway between its inlet end and its outlet end, the passageway being adapted to receive at least a portion of the aerosol through the inlet end as a result of the relative motion of the device and the aerosol, at least a portion of the conduit member defining the passageway being gas-permeable; (b) flow means associated with the conduit member for supplying gas through the gas-permeable portion of the conduit member and into the passageway, and for drawing gas from the passageway through the outlet end of the conduit member; and (c) flow control means operatively associated with the flow means for controlling flow of the gas supplied to and drawn from the passageway in a manner that the gas that is supplied into the passageway is divided into a transporting flow fraction that is drawn from the outlet end and a counterflow fraction that exits through the inlet end and that forces a portion of the aerosol particles from the inlet end, whereby aerosol particles that are received within the passageway but not forced therefrom by the counterflow fraction are transported to the outlet end of the conduit member by the transporting flow fraction.
 9. The device of claim 8 wherein the flow control means includes supply/return flow means for selectively varying the flow rates of the gas supplied through the passageway and the transporting flow fraction, the device being configured so that when the flow of the gas supplied to the passageway exceeds the transporting flow fraction the excess amount of supplied gas flow will exit the passageway as the counterflow fraction, the magnitude of the counterflow fraction thereby being adjustable by the supply/return flow means.
 10. The device of claim 8, wherein the flow means includes a housing surrounding the conduit member, the housing being configured to define a chamber between the conduit member and the housing; the flow means and chamber being configured so that the gas is forced from the chamber into the passageway into the passageway through the permeable portion of the conduit member.
 11. The device of claim 10 wherein the conduit member is substantially tubular; and wherein the housing is substantially tubular.
 12. The device of claim 10 wherein the inlet end of the conduit member is substantially oblong in cross section.
 13. A method for separating particles from an aerosol and transporting those separated particles through a conduit member that defines an interior passageway having an inlet and an outlet, wherein at least a portion of the conduit member is gas-permeable, the method comprising the steps of:(a) creating relative movement between the conduit member and the aerosol so that at least a portion of the aerosol is directed toward the inlet of the passageway of the conduit member; (b) directing an operating gas through the gas-permeable portion of the conduit member; (c) drawing a fraction of the gas that was directed through the gas-permeable portion of the conduit member out of the outlet of the passageway, whereby the remaining fraction of the gas exits the inlet of the passageway; and (d) controlling the flow of the operating gas so that the fraction of the gas that exits the inlet of the passageway forces a portion of the aerosol particles therefrom and so that the fraction of the gas that is drawn out of the outlet of the passageway transports the remaining particles in the passageway out of the passageway outlet.
 14. The method of claim 13 including the substep of heating and cleaning the operating gas before directing it through the gas-permeable portion of the conduit member.
 15. The method of claim 13, wherein the operating gas is a reagent.
 16. The method of claim 13, wherein the operating gas is an inert gas. 